The present invention relates to a brake power control unit for two-circuit brake systems in which a first brake circuit is directly connected to the master brake cylinder without change in pressure and a second brake circuit is connected to the master brake cylinder via the pressure modulating element of the control unit, wherein the brake pressure in the second brake circuit, after a certain changeover pressure determined by the surfaces of a control piston and the force of a spring acting on the control piston has been exceeded, is changed in a relation determined by the control piston, with a locking piston operated hydraulically for neutralizing or locking the pressure change in the controlled brake circuit in case of failure of the uncontrolled brake circuit.
The brake power control unit can be designed as a single-stage or continuously working brake power control unit. The single-state brake power control units, which limit the pressure of the controlled brake circuit to a certain value, are in general called brake power limiters. In these brake power limiters only one end of the control piston has pressure applied thereto, e.g. the controlled pressure. A spring, which together with the surface to which pressure is applied determines the change-over pressure, acts against the force produced by the controlled pressure. In the brake power control unit working continuously the second end of the control piston also has pressure applied thereto, namely the pressure to be controlled. The force produced by this brake power control unit acts in the same direction as the spring force.
In general brake power control units are known. For instance, German Pat. DE-OS No. 24 27 506, published Jan. 9, 1975 shows a continuously working brake power control unit. In this brake power control unit the uncontrolled brake circuit is assigned to the front wheels and the pressure in this first brake circuit acts on the locking piston. The brake pressure to be controlled and the controlled brake pressure act on the control piston from opposite ends. A spring also acts on the control piston in the same direction as the pressure to be controlled. The reduction of the brake pressure in the controlled circuit is determined by this and by the size of the active piston surfaces. If the first brake circuit fails, the pressure of the first brake circuit acting on the locking piston breaks down and this piston is shifted by the controlled brake pressure. Since the locking piston and the control piston are connected to one another via a coupling the valve closed by the control piston is kept open and the pressure can fully build up in the second brake circuit.
However, this known arrangement has a great overall length.